Edward Leigh, the Tory MP, is calling for a change to the Equality Act to prevent a repeat of the case of a Christian housing trust manager who was demoted for airing his views on gay marriage on Facebook.

Adrian Smith, 55, was stripped of his managerial rank at the trust in Trafford, Greater Manchester, and had his salary almost halved after expressing the view that same-sex weddings in churches were “an equality too far”.

The posting, amid a discussion thread on his personal Facebook page, echoed what was Government policy at the time but a colleague took offence, prompting his demotion.

And even though he won a legal challenge against his treatment, the High Court had no power to order his reinstatement.

Mr Leigh fears that teachers and other public servants could be punished and “treated as outcasts” for refusing to promote gay marriage if it becomes law later this year.

It follows legal advice by the human rights specialist Aidan O’Neill QC for the Coalition For Marriage, which campaigns against same-sex unions, concluding that teachers and hospital or prison chaplains would be among those affected.

The former trade minister, who served under Margaret Thatcher and John Major, is bringing forward a 10 minute rule bill in the House of Commons next week, which could open the way for an amendment to the Equality Act to protect such people.

It would classify “conscientious beliefs about the definition of marriage” as a “protected characteristic”, alongside factors such as gender, race, age or disability.

The act makes it illegal to treat people unfairly because of such characteristics.

In his address next week Mr Leigh will accuse the Government, led by his own party, as “arrogant” and “out of touch” on the question of marriage.

“As Aidan O’Neill’s legal opinion suggests, if the Government is successful in redefining marriage, then there are hundreds of thousands of teachers, parents, foster carers, or even hospital and army chaplains who could find themselves being disciplined for their beliefs, just as Adrian Smith was,” he said.

“To think otherwise is out of touch with reality.

“If the Government is serious about protecting those who back the current definition of marriage from being marginalised for their beliefs then it must act immediately to change the Equality Act.

“If it does not then it will opens the door to Christians, Muslims, Jews - and anyone else with a conscientious objection - being disciplined, demoted, or even sacked for backing the current definition of marriage.”

The Coalition is expected to publish its “Equal Marriage Bill” imminently complete with a so-called “quadruple lock” of measures to protect religious groups which do not wish to carry out same-sex ceremonies from being forced to.

But Mr Leigh voiced doubts that any safeguards could withstand a challenge at the European Court of Human Rights.

“The Government might think that any legislation it introduces is bomb proof, but the reality is the UK has a very poor record when tested in the European Courts,” he said.

“According to the Equality and Human Rights Commission, during the period from 1966 - when the UK opted in to the jurisdiction of the ECHR - up to and including 2010, there were 443 judgments relating to the UK.

“In 271 of those cases the ECHR found that the UK had breached the Convention.

“Given the Government’s poor track record of winning in Europe, it would be the height of arrogance to think any legislation will not end up before the European Court, where is stands a good chance of being ruled against.”